The present invention relates to an electric machine and, more particularly, to a DC machine having a stationary magnetic circuit and a rotor, the rotor bearing a winding comprised of a plurality of sections which are electrically connected in series and, at the respective ends of the sections to consecutive segments of a commutator associated with the winding.
The invention finds particular application in respect of direct current motors, and to motor-generators having a single rotor.
In many types of conventional direct current motors, the voltage peaks induced upon commutation are among the leading causes for the wear and damage of the motor brushes and the commutator. In order to minimize these voltage peaks, the section of the winding which is short circuited during commutation must be located in the zone of minimum flux. When the current increases in the rotor, the position of the brushes with respect to the ideal position under load should be modified. This correction, however, generally is possible only in the case of a motor which rotates in a single direction of rotation under constant load.
In order to further reduce the effects of the induced voltage peaks, auxiliary circuits have been proposed, such as resistors provided in parallel with the winding coils. Such resistors may be voltage-dependent resistors (VDR) or be formed of semiconductor elements. Auxiliary circuits have the disadvantage of consuming current and decreasing the efficiency of the motor. Moreover, in most cases they considerably increase the cost of the motor.
In motor-generators having a single rotor, the arrangement of the motor and generator coils on the same rotor makes it possible to avoid shifts between different rotors, and thus avoids mechanical resonances which may otherwise occur upon strong acceleration and deceleration of the rotor. The single-rotor proposal, however, is accompanied by the problem of inductive coupling between the windings. In many typical motor generators, the corresponding sections of the motor winding and of the generator winding are superposed or juxtaposed on the rotor so that the current variations in the motor winding, particularly upon commutation, produce, by induction, parasitic currents and voltages in the generator winding. In machines in which the rotor contains iron, this effect may become even more disturbing than mechanical resonance. Even for those machines using an iron-less rotor, such as the so-called bell or disk-shaped rotor, for instance, inductive coupling constitutes a very substantial disadvantage.